1. Field of the Invention
The present invention relates to occlusive devices (e.g., embolic coils) for implantation within a blood vessel of a body. In particular, the present invention relates to an improved mechanical detachment for an embolic coil delivery system in the treatment of blood vessel disorders.
2. Description of Related Art
Vascular disorders and defects such as aneurysms and other arterio-venous malformations are especially difficult to treat when located near critical tissues or where ready access to malformation is not available. Both difficulty factors apply especially to cranial aneurysms. Due to the sensitive brain tissue surrounding cranial blood vessels and the restricted access, it is very challenging and often risky to surgically treat defects of the cranial vasculature.
Alternative treatments include vasculature occlusion devices, such as embolic coils, deployed using catheter delivery systems. In such systems used to treat cranial aneurysms, the distal end of an embolic coil delivery catheter is inserted into non-cranial vasculature of a patient, typically through a femoral artery in the groin, and guided to a predetermined delivery site within the cranium.
Multiple embolic coils of various lengths, generally approximately 1 cm to approximately 30 cm, and preselected stiffness often are packed sequentially within a cranial aneurysm to limit blood flow therein and to encourage embolism formation. Typically, physicians first utilize stiffer coils to establish a framework within the aneurysm and then select more flexible coils to fill spaces within the framework. Ideally, each coil conforms both to the aneurysm and to previously implanted coils. Each successive coil is selected individually based on factors including stiffness, length, and preformed shape which the coil will tend to assume after delivery.
During implantation, the physician manipulates each embolic coil until it is in a satisfactory position, as seen by an imaging technique such as fluoroscopic visualization, before detaching the coil from the delivery system. It is beneficial for both ends of each coil to remain positioned within the aneurysm after delivery; otherwise, a length of coil protruding into the main lumen of the blood vessel invites undesired clotting external to the aneurysm. After each successive coil is detached, the next coil is subject to an increasing risk of becoming entangled in the growing mass of coils, thereby restricting the depth of insertion for that coil into the aneurysm.
Difficulties may arise due to stretching of the embolic coils during repositioning or attempted retrieval of the coils, especially if the coil becomes entangled and complete insertion of the coil into the aneurysm is not accomplished. If pulling forces applied to a coil exceed its elastic limit, the coil will not return to its original shape. A stretched coil exhibits diminished pushability or retractability, and becomes more difficult to manipulate into an optimal position or to be removed. Moreover, a stretched coil occupies less volume than an unstretched coil, which increases the number of coils needed to sufficiently pack the aneurysm to encourage formation of a robust embolus positioned wholly within the aneurysm. To avoid such problems stretch resistance devices are used, such as that disclosed in U.S. Pat. No. 5,853,418, herein incorporated by reference in its entirety, having a primary coil and an elongated stretch-resisting member fixedly attached to the primary coil in at least two locations.
In order to deliver the vaso-occlusive coils to a desired site, e.g., an aneurysm, in the vasculature, it is well-known to first position a small profile, delivery catheter or micro-catheter at the targeted site using fluoroscopy, ultrasound, or other method of steerable navigation. A delivery or “pusher” wire is then passed through a proximal end of the catheter lumen, until a vaso-occlusive coil coupled to a distal end of the pusher wire is extended out of the distal end opening of the catheter and into the blood vessel at the targeted site. The vaso-occlusive device is then released or detached from the end pusher wire, and the pusher wire is withdrawn in a proximal direction back through the catheter. Depending on the particular needs of the patient, another occlusive device may then be pushed through the catheter and released at the same site in a similar manner.
Several conventional methods are used to detach the wire from the embolic coil once it has been properly positioned at the targeted site in the blood vessel. One known way to release a vaso-occlusive coil from the end of the pusher wire is through the use of an electrolytically severable junction, which is an exposed section or detachment zone located along a distal end portion of the pusher wire. The detachment zone is typically made of stainless steel and is located just proximal of the vaso-occlusive device. An electrolytically severable junction is susceptible to electrolysis and disintegrates when the pusher wire is electrically charged in the presence of an ionic solution, such as blood or other bodily fluids. Thus, once the detachment zone exits out of the catheter distal end and is exposed in the vessel blood pool of the patient, a current applied to the conductive pusher wire completes a circuit with an electrode attached to the patient's skin, or with a conductive needle inserted through the skin at a remote site, and the detachment zone disintegrates due to electrolysis.
One disadvantage of occlusive devices that are deployed using electrolytic detachment is that the electrolytic process requires a certain amount of time to elapse to effectuate release of the occlusive element. This time lag is also disadvantageous for occlusive delivery devices that utilize thermal detachment such as that described in U.S. Pat. No. 6,966,892, which is herein incorporated by reference in its entirety.
Another conventional detachment technique during delivery of a vaso-occlusive device involves the use of fluid pressure (e.g., hydraulic detachment) to release an embolic coil once it is properly positioned, as described in U.S. Pat. Nos. 6,063,100 and 6,179,857, each of which is herein incorporated by reference in their entirety.
The main problems associated with current detachment schemes are reliability of detachment, speed of detachment, convenience of detaching mechanism (e.g., hydraulic detachment requires a high pressure syringe, while electrolytic detachment requires a battery operated box), and length/stiffness of the distal section.
It is therefore desirable to develop an improved mechanical detachment for an embolic coil delivery system that solves the aforementioned problems associated with conventional devices.